1. Field of the Invention
The present invention relates to negative-working diazotype photoreproduction and, more especially, relates to a negative-working diazotype photoreproduction system adapted to utilize conventional diazonium salts in combination with a blocked-coupler which is activated by acid.
2. Description of the Prior Art
Diazotype photoreproduction is of course a standard in the graphic arts. The touchstone of the diazography processes is the light-sensitivity of aromatic diazo salts and the fact that such salts undergo two different types of reactions: [1] replacement or decomposition, in which nitrogen is lost or evolves as nitrogen gas and some other atom or group attaches to the benzene ring in its stead; and [2] "coupling", wherein the nitrogen of the daizo function is retained and the salts react with certain couplable color-forming components, i.e., a "coupler" or "azo-coupling component", to effect formation of an azo dye species.
The photochemical sensitivity of the compounds typically employed in diazotype photographic reproduction ["photoreproduction"] materials resides in the near-ultraviolet region of the spectrum, and is centered about 400 nm [nanometers, one nm being equal to a millimicron or 10.sup.-9 meter]. Photographic diazo processes may be divided into two basic categories: positive-working processes and negative-working or reversal, processes. In the former category, the action of light causes photochemical decomposition of the diazo compound. An image is developed in the unexposed areas by the combination of the diazo compound with a "coupling" component, which is generally an aromatic amine, phenol, or aliphatic compound containing active methylene groups, to form colored oxyazo or aminoazo compounds known as azo dyes.
The positive-working material is imaged by first exposing it through a master transparency or original. The light in the exposure step must supply sufficient energy to destroy the diazo compound in the areas corresponding to the clear background of the original. High-pressure mercury vapor lamps are generally used in performing this step. That part of the diazo coating which is unprotected from the ultraviolet radiation by the image on the original becomes a colorless substance, incapable of coupling to form a dye. The unaffected diazo compound which remains in those areas where the light has not struck is able to form an azo dye by reaction with a coupling component when the medium is made alkaline. Thus, wherever there was an opaque line on the original, a dye-line appears on the copy. Positive-working, diazotype photoreproduction material is generally made alkaline, or pH adjusted, either by impregnating the material with ammonia vapors or passing it through an alkaline developing solution. See generally U.S. Pat. Nos. 1,444,469; 1,628,279; 2,217,189; 2,286,701; 2,429,249; and 2,694,009; German Patentschriften Nos. 56,606; 111,416; 487,247; and 515,205; British Patent Specification Nos. 234,818; 281,604; and 521,492.
In the negative-working or reversal process, a dye is formed in the exposed areas, but not in the areas protected from light. See U.S. Pat. Nos. 2,034,508 and 2,095,408; German Patentschriften Nos. 53,455 and 82,239. Thus a negative, or reversed, copy of the original transparency results. Among the processes employing diazo compounds to produce reversal copies and those employing diazosulfonates. Illustrative thereof is U.S. Pat. No. 2,854,338 to Herrick et al, wherein a neutral or acid photosensitive material is exposed to actinic light and developed in ammonia vapors. Residual diazosulfonate is removed from the unexposed areas and the background cleared by washing in water. In accordance with a more recent process, disclosed in U.S. Pat. No. 3,479,183 to Habib et al, an alkaline imaging material is exposed to actinic illumination to convert the diazosulfonate to an active diazonium compound which combines with a coupling component to provide a reverse dye-image. Since the generation of the imaging diazonium is thus accomplished principally by the incident radiation, powerful light sources must be used for exposing the diazotype material. After imaging, the material is acidified and the unreacted diazosulfonate is light-cleared by exposing it to overall actinic illumination, thereby forming colorless decomposition products of the diazosulfonate to produce a stable, negative dye-image against a clear background. Cf. the U.S. Pat. No. 3,713,825 to Girard; U.S. Pat. No. 1,926,322 to Van der Grinten.
Another negative-working process which utilizes diazo compounds to produce azo dye images is the diazo-oxide mode of photoreproduction.
The basic operating differences between the diazosulfonate and diazo-oxide reversal processes are as follows, reference being made to "Decomposition of o-Hydroxy-Diazonium Compounds by Light," J. DeJonge and R. Dijkstra, Recueil, 67, (1948) pp. 328-342:
[1] The irradiation product of o-hydroxy benzene diazonium compound is a cyclopentadiene carboxylic acid while the irradiation product of benzene diazosulfonate is the structurally isomeric diazo compound itself. As a result of the light decomposition, the practical and significant difference between the negative diazosulfonate process and the diazo-oxide reversal process is the fact that a wide variety of azo dye colors may be produced from diazosulfonate coupler combinations whereas only one azo dye may be produced from each diazo-oxide. The cyclopentadiene-carboxylic acid decomposition product couples much more rapidly than phenolic, naphthol or pyrazalone couplers [see page 335, paragraph 2 of the aforementioned article].
[2] The cyclopentadiene carboxylic acid formed as a result of decomposition under actinic light is a very active coupling component and forms red azo dyes with the diazonium compound.
[3] The fixing or prevention of image formation in the unexposed portions of diazo-oxide coated support requires that there be complete elimination of moisture or adjustment of the coated support to extremely high acid levels. This requirement need not be met with certain of the diazosulfonates.
Nonetheless, it too is well known to this art that negative-working diazography based on either diazosulfonate or diazo-oxide chemistry leaves much to be desired in terms of product imaging characteristics, namely, the diazosulfonate films required long exposure times to effect isomerization of the noncoupling trans-isomer to the reactive cis-form needed to provide an azo dye image. Additionally, the need for impractically lengthy fixing exposures likely shall continue to militate against widespread use of this approach. Insofar as the diazo-oxide based films are concerned, same, in effect, generate coupler in situ through partial decomposition of the diazo moiety. The film is cleared by post-development exposure to destroy background diazo. The diazo-oxide films are also apparently quite limited in density and re-exposure schemes for enhancing image density by several successive re-exposure/cycles have been proposed. Cf. U.S. Pat. No. 4,108,664 to deBoer et al; U.S. Pat. No. 4,094,681 to Habib et al.
Yet another negative-working system features a "vesicular" film which is exposed to actinic radiation to form a latent nitrogen gas image therein from a preselected master image. The nitrogen gas evolves upon the light decomposition of the diazonium salt, and upon storage, the nitrogen would slowly diffuse out of the film. However, upon heating, the nitrogen expands and forms small visible bubbles or vesicles in the film. Thus, vesicular imaging is, in reality, a foaming process of sorts; same is essentially confined to single-component systems applicable to projection imaging, or microfiche/reader uses. Compare, for example, the U.S. Pat. Nos. 3,149,971; 3,555,295; 3,779,768; 3,779,774; 3,841,874; and 3,979,211.
In the U.S. Pat. No. 4,055,425 to Mustacchi, a diazotype material is described comprising a light-sensitive, water-insoluble diazoamino compound, an azo coupler, and an alkaline material, said diazoamino compound, when exposed to ultraviolet light, being reactive with said azo coupler to form a visible dye product only at a pH greater than 7. Such diazotype material can be used for diazotype reproduction processes resulting in either negative or direct positive images. Such material necessitates use of rather exotic equipment and, in the negative-working mode, treatment with an external acid source is mandated, and can be corrosive to the equipment. In addition, the processing entails an extra step in the photoreproduction sequence.
"Thermal" processes are also common to the art; U.S. Pat. Nos. 3,303,028; 3,499,760; 3,695,885; and 3,889,335.
Compare also the negative-working variant disclosed in the U.S. Pat. No. 3,765,895 to Fox wherein free-radical precursors are activated by photodecomposition of a diazonium salt. This patent features the ability of diazoniums to yield a free-radical by homolytic cleavage of the C-N bond, does not entail the formation of azo dyes or ammonia development, and, indeed, such process is operable even in the absence of diazo salt.
Other variations on the theme are suggested by, e.g., U.S. Pat. Nos. 1,857,920, 2,095,408, 2,381,984, 2,416,021, 2,516,931, 2,976,145, 3,140,180, 3,307,952, 3,331,689, 3,563,744, 3,620,740, 3,769,018 and 3,793,033; British Patent Specification No. 975,457.
In view of the above noted technology and in response to the need to develop a simple negative-working diazography process a number of formulations were discovered which are disclosed in U.S. Patent Application Ser. No. 66,401, of E. W. Bennett, filed on Aug. 14, 1979, now U.S. Pat. No. 4,252,884, the disclosure of which is herein incorporated by reference.
The primary components of this formulation comprise (1) at least one diazonium compound, (2) an inactive blocked-coupler and (3) a specifically defined photolabile, or light-sensitive acid progenitor.
In a preferred embodiment, the diazo compound and light-sensitive acid progenitor constitute the same compound. This is achieved when the diazonium compound is a diazo salt comprising a complex anion of an inorganic Lewis acid. Thus, in this embodiment the diazonium compound functions as either a reactant source for the formation of an azo dye species upon reaction with an activated coupler, or as an acid generating compound wherein a controlled amount of light decomposes the diazonium salt thereby releasing nitrogen and a Lewis acid.
The above described formulation is employed in a negative-working imaging process wherein the formulation is typically provided in the configuration of a film. The film is subjected to an imagewise exposure of light in a manner sufficient to generate catalytic amounts of acid from, for example, the photolabile diazonium salt. Preferably aided by heating, the liberated acid reacts with the inactive blocked-coupling agent to unblock its functional group, and impart to the unblocked coupler the potential to react with the diazonium salt. The reaction between the diazonium salt and the coupling agent is induced in a developing stage wherein the film is subjected to an alkaline environment, preferably, at elevated temperatures, to form the azo dye.
The unreacted diazonium compound is preferably decomposed in a clearing step by exposing the developed film to actinic light, e.g., for about 60 seconds.
While the above processes and formulations employed therein have many advantages over the prior art certain difficulties have been observed. For example, when films employing certain of such formulations are cleared by overall exposure to light the color of certain of the azo dyes is often changed in a drastic fashion. Thus, yellows become purple, sepias become blue and so forth.
Furthermore, often within a few days after the finished print is obtained the transparency of the image in the minimum density areas thereof is reduced substantially. This loss in transparency is commonly observed when a hexafluorophosphate diazonium salt is employed in the diazography formulation in conjunction with a clearing step by overall exposure to light. More specifically, it has been observed that large amounts of acid are liberated from the diazonium salt during the clearing step. The acid environment created threby adversely affects the color of acid sensitive dyes. This is referred to as acid discloration.
The cause of the loss in transparency of the image over a period of several days from the time when the print is obtained has, it is believed, heretofore been unknown. However, after considerable experimentation it has been discovered that the loss in transparency is believed to be due to the formation of ammonium dihydrogen phosphate bloom which forms as a result of the reaction between acid liberated during the clearing step and the ammonia remaining in the film after the developing stage which employs moist ammonia vapors.
Thus, it would be a distinct advantage if the acid liberated during the clearing step could be scavenged before acid discoloration occurs and if the bloom, i.e., formation of ammonium dihydrogen phosphate, could be substantially reduced or eliminated.
The selection of an appropriate acid scavenger is complicated by the fact that it must be substantially inactive during the imaging step otherwise the acid liberated thereby would be inactivated, and the unblocking of the blocked-coupler by acid cleavage would thereby be reduced substantially. In addition to the above requirements, an acceptable additive should be preferably insensitive to moisture, colorless, compatible with the film matrix material, and soluble in ketone and alcohol solvents.
Thus, the search has therefore continued for ways to improve the image and color stability of certain diazography formulations. The present invention was developed as a result of this search.
Accordingly, it is an object of the present invention to improve the acid color stability of certain negative-working diazography formulations described herein which are subjected to a clearing step.
It is a further object of the present invention to improve the image stability of certain diazography formulations described herein which are subjected to a clearing step.
These and other objects and features of the invention will become apparent from the claims and from the following description when read in conjunction with the accompanying drawings.